A Multiple-Input Multiple-Output (MIMO) system is capable of obtaining frequency efficiency proportional to the number of transmit antennas in an environment of rich scattering and of obtaining very high wireless transmission rate in a condition of not sacrificing frequency resource and transmit power, and thereby it is widely concerned.
An Orthogonal Frequency Division Multiplexing (OFDM) system modulates transmit symbols onto orthogonal narrow-band sub-carriers so that the symbols transmitted on each sub-carrier is only subject to flat fading and thus design for receivers is largely simplified, which is an efficient solution to high rate wireless data transmission under multipath fading channels.
It is assumed that the channel is flat in an existing MIMO structure, but the actual wireless communication channel is frequency-selective. Therefore, combination of MIMO and OFDM will meet requirement of MIMO on channel selection and is one of advanced wireless communication solutions in future.
MIMO has two basic space-time structures. One is space-time coding and the main object thereof is to obtain coding gain. The other is space multiplexing and the main object thereof is to obtain data rate gain. There are three basic structures in space-time coding schemes: it is referred to as space-time block coding if space diversity is obtained by jointly coding the transmitted information symbol stream in space and time domains; it is referred to as space-frequency block coding if space diversity is obtained by jointly coding the transmitted information symbol stream in space and frequency domains; it is referred to as space-time-frequency block coding if space diversity is obtained by jointly coding the transmitted information symbol stream in space, time and frequency domains. Although diversity in space, time and frequency domains can be obtained by space-time-frequency coding, the space-time block coding and the space-frequency block coding (it is recorded as space-time/space-frequency block coding) is more efficient in an actual application since the space-time-frequency coding relatively has high complexity and low flexibility.
The existing technical solution is described as follows taking example for a two-transmit-two-receive MIMO-OFDM system.
FIG. 1 and FIG. 2 respectively illustrated structures of transmitter and receiver for space-time block coding or space-frequency block coding in an existing MIMO-OFDM system. In the transmitter, the transmitted source bit sequence is mapped to a modulated constellation to generate a modulated symbol sequence after channel coding and interleaving. The modulated symbol sequence is space-time block coded (space-frequency block coded) to generate two sets of symbol streams respectively corresponding to transmit antenna 1 and transmit antenna 2. After pilot symbols are inserted according to pilot pattern, the respective symbol streams of the transmit antennas are inverse fast fourier transformed (IFFT) to generate OFDM symbols, each of which has circle prefix inserted at the head to generate a spread OFDM symbol. A number of spread OFDM symbols form a time slot, and a pseudo random synchronization sequence is inserted prior to each time slot for timing synchronization. A number of time slots form a transmission frame of minimum transmission unit.
In the receiver, the received symbols streams are first frame synchronized, time slot synchronized and symbol synchronized based on synchronization sequences. The synchronized spread OFDM symbols are fast fourier transformed (FFT) after the circle prefix is removed. A frequency domain pilot is extracted from the OFDM sub-carrier for channel estimation. The estimated channel frequency response is assistant for decoding of remain data symbols by a space-time block coding decoder/a space-frequency block coding decoder. The generated symbol streams are demodulated, deinterleaved and channel decoded to generate an estimated source bit sequence.
The main differences between the space-time block coding and the space-frequency block coding are described as follows in FIG. 3 and FIG. 4.
As shown in FIG. 3, the pre-transmitted modulated symbols S1 and S2 are coded by a space-frequency block coder to generate a symbol stream S1, S1*, S2 and −S2*. Then, S1 and S1* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 1; S2 and −S2* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 2. As such, the next set of pre-transmitted modulated symbols S3 and S4 are coded by the space-frequency block coder to generate a symbol stream S3, S3*, S4 and −S4*. Then, S3 and −S4* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 1; S4 and S3* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 2.
As shown in FIG. 4, the pre-transmitted modulated symbols S1 and S2 are coded by a space-time block coder to generate a symbol stream S1, S1*, S2 and −S2*. Then, S1 and −S2* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 1; S2 and S1* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 2. The pre-transmitted modulated symbols S3 and S4 are coded by the space-time block coder to generate a symbol stream S3, S3*, S4 and −S4*. Then, S3 and −S4* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 1; S4 and S3* are mapped to adjacent sub-carriers of the same OFDM symbol and transmitted by antenna 2.
It is assumed that the channel matrix is constant between adjacent coded OFDM symbols in the space-time block coding designed based on orthogonal signals. However, the symbol interval of the OFDM system largely increases compared with a single carrier system with the same bandwidth so that the condition that the channel matrix is constant between adjacent coded OFDM symbols is difficult to be achieved, and thus the performance of space-time block coding is degraded. Particularly, since the future mobile communication system supports users with higher frequency band and higher moving speed, signals subject to fast fading will badly degrade performance of the space-time block coding. Similarly, the channel matrix is constant between adjacent coded OFDM symbols in the space-frequency block coding designed based on orthogonal signals. However, the actual channel is usually frequency-selective, so that the condition that the channel matrix is constant between adjacent coded OFDM symbols is difficult to be achieved, and thus the performance of space-frequency block coding is degraded.
FIG. 5 and FIG. 6 respectively illustrate comparison of performances of the space-time block coding and the space-frequency block coding at different delay spreading and different moving speed. When delay spreading of channel is less, the same bit error rate (BER) performance is obtained in a range of moving speed of 5-120 kmph for the space-frequency block coding (SFBC). Even when the moving speed is up to 500 kmph, performance as good as at a low speed can be obtained for the space-frequency block coding if an ideal technique for removing interference between carriers is used. When delay spreading of channel is greater, performance will be badly degraded in a variety of ranges of moving speed for the space-frequency block coding, which attributes to channel frequency-selection caused by greater delay spreading resulting in non-constant between adjacent carriers of the space-time block coding. Similarly, good BER performance can be obtained for the space-time block coding at a low moving speed of 5 kmph no matter how much the channel delay spreading is. However, in a range of moving speed of 120-150 kmph, the performance of the space-time block coding is badly degraded, which attributes to fast channel fading caused by greater Doppler shift resulting in non-constant channel between coded symbols in the time domain.
In one word, the space-time coding can operate at frequency-selective channels, but it is subject to effect on orthogonality by Doppler shift, which badly degrades performance; the space-frequency coding can operate at Doppler shift channels, but it is subject to effect on orthogonality by frequency-selection, which badly degrades performance.
Therefore, there is a certain limit in the environment of any existing space-time coding scheme.